Understanding Weather – Not Predicting – 2 March 2012

What happened?The disparate scene which has been hovering for some while is back in the weather saddle. While winter reigns across the Southern Ocean a healthy summer synopsis controls the Indian Ocean. A summer remnant is present above Angola.Analysing this draws attention to a long standing vortex area south of Africa between 50 and 60oS and ensuing cold front developments and movement. Two cold fronts with upper air troughs passed the Cape in this last week and there are two more to come next week. In one way this is not unusual, but as it is only late summer, this frontal presence is a couple of months ahead of regular synoptic schedule. From the Mozambique Channel eastward beyond Australia and into the Tasman Sea a trough line is identified with two major vortex cores. This is the Inter-Tropical Convergence Zone (ITCZ) satisfactorily in place where it should be for the time of year. A weak, convergent air pattern survives above western Angola and westward above the warm Kunene flood plume. The result, in some disagreement with last week’s outlook, is limited rain for our weather world. A key divergence was that Tropical Cyclone Irina drifted west, not south, so entering the Mozambique Channel before making its way mid-channel southward. Its ring of high altitude outflow blocks the mid-level anticyclone above Zambia and the Congo thus preventing more moisture to reach Namibia.The high pressure area ridging in from South Africa should have acted as an engine to drive more moisture to Namibia but its effect was diluted by the low-pressure cyclone in the Mozambique channel.Now these few days are part of that key period in the seasonal pattern where rain-making activity should be at its maximum (remember 2011 among others?). That there seems to be a reluctance for the whole summer pattern to begin easing northward does offer an indication that there is still “more to come”. Just as much as the strong 1040+hPa, intense (ridge at 45000 feet) anticyclone in mid Pacific, 40 to 50oS, maintains a La Nina control for the west and mid equatorial Pacific.What’s coming?This pattern takes us into the new week. The moist status quo persists close to the northern border, one cold front departs while the next one approaches Cape by weekend and departs during Monday. Irina is expected to take southern Mozambique by storm during Saturday; here lies dissent.One forecast sees a drift westward with decay over land due to the lack of warm sea moisture to fuel it, though the decaying air-flow brings dry air into our middle levels until mid-week. The other view sees a decline on the coast and then a departure in tandem with the Monday cold front activity by then close to that maritime area. Despite this doubtful outlook, the prospect of some input from the Angolan air-mass, supported by the flood plume presence, offers some showers for the north and central to western parts, only. The southern half of the country will remain mostly dry, but windy.